def remove_duplicate(sentence):
  out = sentence[0]
  dupes = ''
  last = sentence[0]

  for letter in sentence[1:]:
    if letter == last:
      dupes += letter
    else:
      out += letter
    last = letter
  return out

assert(remove_duplicate('hello') == 'helo')
assert(remove_duplicate('hhhhheeeeelllllooooo') == 'helo')